Method of coating metal surfaces with phosphate



United States Patent METHOD OF COATING METAL SURFACES WITH PHOSPHATE Montagu Hyams, London, England, .assiguor, by direct and mesne assignments, to Neilson Chemical Company, Detroit, Mich, a copartnership No Drawing. Application 'June 9,'-1953 SerialNo. 360,597

5 Claims. (Cl. 148-6.15)

This invention relates to the surface treatment of'metal by the chemical action thereon'of a water-miscible 'surfacetreating medium.

The chemical surface treatment of metal -is normally carriedout bycontacting the surface of metal with alarge volume of treating liquid, applied by immersion or -by spraying. cleaning processes, that a large volume of liquid to "be used in subsequent treatmentsbecomes contaminated during the treating process. Moreover, especially in'coating processes, supersaturation or desired pH change at the liquid/metal interface takes place but slowly 'due to "the washing action of the .ever changing liquid 'at the'interface. Another disadvantage of this type 'of treatment'is that, where the treating liquid is to beapplied hot, '11 large volume of liquid must be maintained in heated condition.

These disadvantages may be overcome by applying the surface treating medium -to the metal surface as a film and allowing the film to remain on the metal surface until the desired degree of treating action has been achieved. Such film processes as carried out hitherto have had limited application zhoweverzand-for many applications have not been at all satisfactory. It has normally been found necessary to operateat room =,temperature or, if elevated temperatures ,are who used, tocontact the metal only for short periods and with dilute solutions, to prevent prematureArying-of the film. .The use of dilute solutions has meant insufficiency of [active material at the metal surface and .of :such short heating periods that the process usually is incomplete. 7I11-Sl1Ch processes the film quickly dries, the chemical a-ction ceases and there is left upon the metal surface a difiicultly removable or washable residue. Ifthe surface undergoing treatment is of ferrous metal, there is very greatftendency for the drying surface to oxidise :and rust under theinfluence of the atmosphere.

This involves .the disadvantage, especially in v An objectof the present invention is to providera proction as set out above are fully obtained, and in which high concentrations of liguidmayibe applied tit-elevated temperature until the treating action has reached the desired degree of completion without the attendant disadvantages normally associated with film type processes as set out above.

According to the present invention, a process of treating the surface of metal by the chemical act-ion thereon of a water-miscible surface treating medium comprises providing upon said surface a film of said medium and contacting the film so provided with steam having a temperature in excess of 180 F.

The surface treating process may be a coating process wherein the metal surface is provided with an adherent non-metallic coating, such as an insoluble metal phosphate coating. In such processes, the presence of a polyvinyl methyl ether or of a polyoxyethyleneoxypropylene 2,845,376 Patented July 29, 1958 '2 glycol or ether thereof in the coating medium may facilitatecoating'formation.

The said film may be applied to the surface undergoingtreatment by immersing the surface in, why Howing or spraying onto said surface, a liquid or powdered or paste-like surface treating medium. Application of steam '.to the film may involve immersing thefilm in a bath-of steam ordirlecting steam in spray'orjetform-onto theifilm, orboth. I

According to one feature of the invention, the metal surface bearing the film of surface treating medium, especially a coating liquid, :is immersed in a bath -of steam, asfor example in a "conventional bottle sterilising plant, :untilzthe "treating action, for example the coating process, isisubstantiallycomplete.

According .to another feature '"of the invention, the film of surface treating medium is sprayed with steam continuously or intermittently, until the treatment is "subsantially complete. When the steam is sprayed "intermittently,:it is'preferred to 'renewz thefilm afterat least one aotherthanthe last spraying period. 7

The invention finds .particularly 'useful application in the phosphate coating of metals, especially of ferrous metals, and will be illustrated 'with this field of application irrmind.

Thezfollowing examples illustrate the invention.

.Example I .A coating :solution was prepared from 175 parts by weight of zinc-dihydrogenphosphatedihydrate, 30 parts by weight:of42 -B. nitric-acid, 1.5 parts zby'weight 'of copper carbonate and 295 parts by weight of water. Iron wire to be drawn was -uncoiled from a roll and given a thin film of. the cold .coating solution by passing through rollers provided with .a film of the coat-ing material. The wire issuing from the rollers was passed through a narrow tube which was thermally lagged and in which steam, at substantially atmospheric pressure was slowly circulated counter-current to the .direction of travel of the wire. The rate of travel of the wire through the steam-filled tube was such that the residence time of .a given section of theWire in the tube was 30 .secs.- The treated wire emerging from the steam-filled tube was passed through a water spray rinse zone.and wasffound to have an excellent coating of zincphosphate forffacilitating-thesubsequent drawing process. 1

Example II =Anunrber of rusty steel bolts were loaded into aperforated 'barrel rotatable upon "its horizontal ;axis. The barrel was immersed in acold solution formed by dissolving one part by volume of, orthoph'osphoric acid in two-parts by volume of water. The barrel was immersed in the solution for -5-10sec0nds -and 'thenre moved: and allowed to drain. The barrel was then rotated about its horizontal axis in a loosely enclosing sheath iwliile ssteam from e20. p. s. =i. source -was continuously led into the inside of the rotating barrel and escaped through the perforations and out through the space between the wall of the barrel and the enclosing sheath. After three minutes of such treatment the bolts had become coated with a uniform coating of iron phosphate and were free from rust.

Example III A steel panel, somewhat imperfectly freed from rust and grease, was treated in a conventional spray plant by means of a decimolar solution of sodium dihydrogen orthophosphate sprayed onto the panel at a temperature of F., the solution also containing a trace of wetting agent. After two minutes spraying the panel was imperfectly coated with iron phosphate. In another processing run, the panel was treated in similar fashion except that after every 20 secs. of processing the sprays of liquid were cut off and the work was sprayed with steam from a 20p. s. i. source. After two minutes operation in all, the panel was coated uniformly over its whole surface, the steam treatment having activated the metal surface, especially the more difficultly coated portions of the surface.

Example IV A refrigerator cabinet of ferrous metal was supported horizontally about its central longitudinal axis two-thirds immersed in a conventional cold phosphating solution maintained at around room temperature and pH of 2.25 and containing, in g. p. l. of bath, 19.5 Zn, 23.2 P 19.3 Na and 69.8 N0 The solution also contained 2% by weight of polyvinyl methyl ether. The refrigerator cabinet was slowly rotated about its horizontal axis while supported in the processing solution, and near the top of its travel out of the solution the cabinet was subjected to the action of steam sprayed onto it from a 20 p. s..i. source. After five minutes of such treatment the processing bath had not increased in temperature by more than 2-3 F. and the cabinet was covered with a fine uniform coating of zinc phosphate of much finer grain and uniformity than when the mild steam heating was not applied. It is believed that this was due partly to the heat facilitating the seeding out of tertiary zinc phosphate, and to the polyvinyl methyl ether which was reversibly precipitated by the heating action facilitating the precipitation of the seeded zinc phosphate and preventing clogging of the surface with too rapidly precipitated and loosely adherent zinc tertiary phosphate. Instead of polyvinyl methyl ether, a polyoxypropyleneoxyethylene glycol of around 40 C. flocculation temperature may be used.

Example V Food cans of steel are brushed with a coating solution prepared as described in Example I. The coated cans are led suspended from a travelling chain in zig zag manner through the chamber of a conventional bottle sterilising machine maintained at a temperature in the chamber of around 185 F. Upon emerging from the machine after having travelled therein for two minutes they have an excellent adherent surface coating of insoluble metal phosphate.

I claim:

1. Process of providing upon the surface of metal an adherent coating of insoluble metal phosphate through the chemical action upon said surface of an acidic, watermiscible phosphatising liquid, which comprises providing upon said surface a film of said liquid and subsequently, while said film is still in the liquid state, spraying steam having a temperature in excess of 180 F.-in substantially continuous manner upon the film so pro vided until the desired degree of coating has been attained, said film being replenished with coating liquid at least once during the period in which steam is applied thereto.

2. Process of providing upon the surface of metal an adherent coating of insoluble metal phosphate through the chemical action upon said surface of an acidic, watermiscible phosphatising liquid, which comprises providing upon said surface a film of said liquid and subsequently, while said film is still in the liquid state, spraying steam having a temperature in excess of 180 F. intermittently upon the film so provided until the desired degree of coating has been attained, said film being replenished with coating liquid after at least one period of spraying with steam other than the last such period.

3. Process of providing upon the surface of metal an adherent coating of insoluble metal phosphate which comprises successively immersing the surface in and withdrawing the surface from, a bath of phosphatising solution a plurality of times and subsequently, while said film is still in the liquid state, spraying steam having a temperature in excess of 180 F. upon the metal surface between successive immersion periods.

4. Process of providing upon the surface of metal an adherent coating of insoluble metal phosphate which comprises successively immersing the surface in and withdrawing the surface from, a bath of phosphatising solution a plurality of times and subsequently, while said film is still in the liquid state, spraying steam having a temperature in excess of F. upon the metal surface between successive immersion periods, said phosphatizing solution containing a compound which is precipitated upon heating, and redissolved upon cooling, the solution and is selected from the group consisting of polyvinyl methyl ethers, polyoxyethyleneoxypropylene glycols and polyoxyethyleneoxypropylene glycol ethers.

5. Process of providing upon the surface of metal an adherent coating of insoluble metal phosphate which comprises applying a phosphatizing liquid to a metal surface to form a film thereon, and subsequently, while said film is still in the liquid state, applying steam to said film to complete the phosphate coating action.

References Cited in the file of this patent UNITED STATES PATENTS 753,610 McDowell Mar. 1, 1904 1,254,264 Oeschger Jan. 22, 1918 1,355,074 Cleveland Oct. 5, 1920 1,529,168 Cleveland Mar. 10, 1925 1,696,036 Heinzman Dec. 18, 1928 1,761,186 Baker June 3, 1930 1,837,159 Fellows Dec. 15, 1931 1,949,713 Gravell Mar. 6, 1934 2,203,063 Schueler June 4, 1940 2,228,836 .MacQuaid Jan. 14, 1941 2,254,980 Simmons Sept. 2, 1941 2,255,493 Pfalzgratf Sept. 9, 1941 2,334,177 Dishauzi Nov. 16, 1943 2,479,423 Snyder Aug. 16, 1949 2,516,008 Lum July 18, 1950 2,516,685 Douty et a1 July 25, 1950 2,529,762 Brisse Nov. 14, 1950 FOREIGN PATENTS 673,638 Great Britain June 11, 1952 

5. PROCESS OF PROVIDING UPON THE SURFACE OF METAL AN ADHERENT COATING OF INSOLUBLE METAL PHOSPHATE WHICH COMPRISES APPLYING A PHOSPHATIZING LIQUID TO A METAL SURFACE TO FORM A FILM THEREON, AND SUBSEQUENTLY, WHILE SAID FILM IS STILL IN THE LIQUID STATE, APPLYING STEAM TO SAID FILM TO COMPLETE THE PHOSPHATE COATING ACTION. 